The present invention relates to compositions for gypsum board, and more specifically to liquid gypsum set accelerators used in the manufacture of gypsum board.
Gypsum board (e.g., wallboard or drywall) is commonly used in the building industry for the construction of walls and other structures. Gypsum has a number of desirable physical properties, including fire resistance, thermal and hydrometric dimensional stability, compressive strength, and neutral pH. A major ingredient in the manufacture of gypsum wallboard is calcium sulfate hemihydrate, commonly referred to as “calcined gypsum” or “stucco.” Stucco is prepared by drying, grinding, and calcining raw gypsum (i.e. calcium sulfate dihydrate). The raw gypsum is dried in a kiln to remove free moisture and ground in a mill to a desired fineness. Dried, fine-ground gypsum is also referred to as “land plaster.” The land plaster is calcined by heating to undergo a dehydration reaction that produces calcium sulfate hemihydrate (stucco) and water vapor, as described by the following chemical equation:CaSO4.2H2O+heat→CaSO4.½H2O+1½H2O
The calcined gypsum, or stucco, has the desirable property of being chemically reactive with water and will “set” rather quickly when the two are mixed together, to form a matrix of calcium sulfate dihydrate crystals. The hydration or setting reaction is the reverse of the calcination reaction, and proceeds according to the following chemical equation:CaSO4.½H2O+1½H2O→CaSO4.2H2O+heatAs shown by the chemical equation, the setting reaction is exothermic and is accompanied by the release of heat.
Gypsum wallboard comprises a gypsum core sandwiched between two sheets of facing material (e.g., paper or fiberglass). The gypsum core is commonly produced from a gypsum slurry that is prepared as a mixture of dry and wet ingredients. The dry ingredients consist primarily of stucco, and may include, but are not limited to, any combination of fiberglass, set accelerators, functional fillers (e.g., vermiculite), potash, crystal modifiers (e.g., boric acid), binders (e.g., natural polymers such as starch), and/or other ingredients as are known in the art. The wet ingredients consist of water and may include paper pulp (the “pulp paper solution”), and/or one or more additional components that are known in the art, including, potash, dispersants, set retarders, polymers, wax emulsion, silicone, surfactants, and thickening agents. If present, the pulp paper solution along with the gauging water provide a significant portion of the water that forms the gypsum slurry of the core composition of the wallboard. The dry ingredients, gauging water, and the pulp paper solution contain the basic chemical components of a piece of wallboard.
The commercial manufacture of gypsum wallboard typically involves a continuous, high-speed process. The dry and wet ingredients, gauging water, and pulp paper solution are combined in a mixer (e.g., a pin mixer) to create a fluid mixture or slurry. The gypsum slurry exits the pin mixer into a canister and is discharged through an outlet chute or “boot,” which spreads the slurry onto a moving, continuous sheet of bottom facing material (back). A moving, continuous sheet of top facing material (face) is placed on the slurry, so that the slurry is sandwiched between the top and bottom facing materials to form the gypsum board. The board passes through a forming station which forms the wallboard to the desired thickness and width. Although the facing material is described as paper, other materials known in the art may be used as a facing material, such as fiberglass mat.
The board travels along a belt line for several minutes, during which time the board stiffens and “sets” as the stucco and water rapidly undergo a hydration reaction to form crystals of calcium sulfate dihydrate, as described above. The boards are cut to a desired length and fed into a large, continuous kiln for drying. During the drying process, the excess water (free water) is evaporated from the gypsum core while the chemically bound water is retained in the newly formed gypsum crystals.
The time required to complete the setting reaction is generally governed by the speed and length of the production line and typically ranges from about 5 to 15 minutes. The rate of the stucco hydration reaction can vary based on factors such as the method of calcination, slurry temperature, and the type of raw gypsum (e.g., natural or synthetic). Gypsum set accelerators and retarders are commonly added to the gypsum slurry to adjust the rate of the setting reaction and ensure that the gypsum slurry has sufficiently set and hardened to allow the board to be cut, and that the hydration reaction is substantially complete before drying in the kiln. The ratio between the accelerator and retarder, as well as the quality of these agents, along with other factors such as mix temperature and the water to stucco ratio will affect the hardening rate of the stucco.
A common accelerator used to reduce the set time of the gypsum slurry is calcium sulfate dihydrate, which is produced as a dry powder from crushed and dried gypsum. The gypsum may be synthetic or naturally occurring. The raw gypsum is ground in a high energy mill to produce a fine gypsum powder that is used as a catalyst and crystal seed source for gypsum hydration and crystal nucleation during gypsum board manufacture.
Land plaster based accelerators are susceptible to degradation and reduced potency before they can be added to the gypsum slurry. The process of grinding the gypsum to produce land plaster generates heat, which may cause partial calcination and can require intermediate cooling stages that interrupt the manufacturing process. Calcination of the accelerator may also occur upon mixing with stucco that is still warm from the calcination processor in elevated process temperatures. Humid conditions also lead to a loss in potency. The exposure to moisture may cause clumping of the calcium sulfate dihydrate particles and may also reduce crystallinity. The loss in potency can cause inconsistent hydration reaction rates and may increase the amount of the accelerator required to achieve the desired setting time. Furthermore, the limited shelf life of the accelerator increases the difficulty in maintaining a store of accelerator to support a continuous production process.
Land plaster based accelerators can also be difficult to disperse in the gypsum slurry, and can cause problems in the manufacturing process. Fine dry powders are susceptible to caking and clumping during storage, and can also form insoluble agglomerates when introduced to water. To ensure that the accelerator is adequately dispersed in the gypsum slurry, conventional accelerators are typically introduced to the manufacturing process at the pin mixer, with the stucco and other dry ingredients. However, the accelerator begins to promote the setting reaction in the mixer and can cause lumps to form in the gypsum slurry. These lumps can clog and obstruct the flow of the gypsum slurry through the equipment, may result in uneven discharge and spreading of the gypsum slurry onto the facing material, may interfere with the forming of the gypsum board, and may cause breaks in the paper facing material which can create process upsets and downtime. Thus, the use of conventional accelerators often requires interruption of manufacturing for periodic cleaning of the mixer and other equipment, resulting in substantial process downtime.
Therefore, it would be desirable to provide a gypsum set accelerator that has reduced susceptibility to degradation and loss of potency, and that reduces process upset and the need for equipment maintenance and downtime. It would also be useful to minimize the dwell time of the accelerator while the gypsum slurry is in the equipment, to allow the majority of the setting reaction to occur after the gypsum slurry has been applied to the facing material and the board is formed. Thus, it would be desirable to provide a gypsum set accelerator that is more easily dispersed in the gypsum slurry, to allow the accelerator to be introduced into the manufacturing process downstream of the mixer—e.g., at the canister or boot.